In recent years, filters using a differential (balanced) type signal for a receiving signal are used in wireless communication devices such as cell phones to suppress a common-mode noise in a high-frequency circuit. For example, an Acoustic Wave (AW) device where a circuit combining a balun and ladder-type filters is connected between a balanced terminal and an unbalanced terminal is known as disclosed in Japanese Patent Application Publication No. 2007-312324 (Document 1).
In the acoustic wave device disclosed in Document 1, a filter is directly connected to an input terminal. Thus, an input impedance of the filter is fixed. Therefore, a design flexibility is lost, and it is difficult to downsize the filter.